This invention relates to a process for the preparation of para-amino diphenylamine by means of the catalytic hydrogenation of para-nitroso diphenylhydroxylamine.
The hydrogenation of para-nitroso diphenylhydroxylamine to para-amino diphenylamine is well known. According to a process described in German patent application disclosure no. 1,941,008, the nitroso compound may be hydrogenated either in a liquid mixture with a hydroxylic solvent, such as water, or a primary or secondary alcohol, or in the gaseous phase. The catalysts used in the process are combinations of two or more of the heavy metals, iron, manganese, cobalt, copper, nickel, silver, cerium and lead, in the form of their oxides, hydroxides or carbonates. Hydrogenation may be carried out at temperatures from 100.degree. to 250.degree. C., preferably under elevated pressure, and is indicated to result in the desired para-amino diphenylamine, with a yield of 74 to 93%. However, the yield is not of analytically pure product, but of crude product, after removal of the solvent. Therefore, one must assume that by-products, such as products formed by hydrogenation of the nucleus, are included in the product yield.
According to the process of British Pat. No. 1,296,211, the para-nitroso diphenylhydroxylamine may be charged as its alkali derivative and hydrogenated at temperatures between room temperature and 120.degree. C., in an aqueous medium, in the presence of a hydrogenation catalyst. Metals of Group VIII of the periodic system, for example, nickel, cobalt, ruthenium, palladium, or platinum, may be used as catalysts, which, if desired, may be applied to an inert carrier. The quantity of catalyst is from 0.1 to 10, preferably 0.1 to 2%, by weight. Hydrogenation may be carried out in the usual manner, at temperatures between room temperature and 120.degree. C., and preferably under elevated pressure. Also, it is desirable to utilize an inert, organic solvent, which is partly or completely miscible with water, as such methanol, ethanol, n-butanol, or dioxane, or an inert, organic solvent which is not miscible with water, such as toluene, xylene or monochlorobenzene. In the process of the British patent as well, the yields of para-amino diphenylamine are in the range from 40 to 88% (crude product). According to British Pat. No. 1,304,525, when alcohols are used as solvents, good yields are obtained only in the case of propanol, isopropanol, n-butanol, and isobutanol (71 or 83% of theoretical crude product), whereas in the case of other alcohols, such as ethanol, n-amyl alcohol, and isoamyl alcohol, the yields are substantially lower (32.5, 45.8 or 41.6% of theoretical crude product).
The reduction of para-nitroso diphenylhydroxylamine to para-amino diphenylamine by means of the catalytic transfer hydrogenation is known from German patent application disclosure no. 2,715,785. Hydrogenation may be carried out in the presence of a catalyst based on a noble metal of Group VIII of the periodic system. Formic acid or a formate, a phosphorus compound with at least one hydrogen atom bonded immediately to the phosphorus, or hydrazine, which may contain up to two methyl groups, serve as hydrogen donors. The catalyst is used in quantities of up to 25% by weight, preferably up to 10% by weight of precious metal, based on the substrate. Preferably, the reduction is carried out in a mixture of water and tetrahydrofuran. The high quantity of catalyst notwithstanding, the yield of para-amino diphenylamine is only between 70 and 90%.
From the foregoing it is quite apparent that there exists a need for an improved process for the production of para-amino diphenylamine which provides for the production of such a compound in relatively high yields.